The invention relates to a control circuit and method for controlling the power supplied to an electric device. The invention has particular relevance to a dimmer switch for an LED light source.
In recent years, LED light sources have been replacing incandescent bulbs, fluorescent bulbs and other light sources in a variety of lighting applications due to their efficiency, small size and high reliability. However, using a dimmer switch designed for use with one of those conventional light sources with an LED light source may result in sub-optimum performance.
A dimmer switch for an incandescent light source typically uses a phase-cut thyristor arrangement to control the duration of the part of each AC mains half-cycle that is supplied to the incandescent light source. In particular, a user can adjust the point in time in the AC mains half-cycle when the thyristor switches on; the earlier the thyristor is switched on, the brighter the incandescent light source. FIG. 1 shows the output voltage waveform of an example of a typical phase-cut thyristor arrangement for a single cycle of a 50 Hz AC mains input voltage. In this example, during the first 4 ms of each AC half-cycle no current is applied to a thyristor gate so that no power is output to the light source during this time. By adjusting the time when current is first applied to the thyristor gate, the RMS output voltage/current is increased or decreased such that the brightness of the incandescent light source is also increased or decreased. The thyristor switches on when a gate current exceeds a threshold value, but will then remain conducting until the load current through the thyristor drops below a threshold value for sufficient time for the thyristor to return to a blocking state, regardless of the level of the gate current, which typically drops to a very low value after the thyristor starts conducting, to conserve power.
Conventional thyristor-based dimmer switches work well with predominantly resistive loads, such as incandescent light sources, because following the thyristor switching on in an AC mains half-cycle, the load current only drops low enough for the thyristor to switch off at the end of the AC mains half-cycle. However, thyristor-based dimmer switches do not work satisfactorily with loads having a large capacitive component, such as drive circuits for some LED light sources, because following the thyristor switching on, a large initial load current is followed by a low load current that causes the thyristor to switch off. Given the rapid response time of LEDs, the switching off of the thyristor can result in the LED light source temporarily switching off before a gate current is applied during the next AC mains half-cycle, to cause the thyristor to conduct again. This switching on and off may be perceived as an undesirable flickering of the LED light source.
There is an analogous problem in controlling electric motors using a phase-cut thyristor arrangement. Instead of LEDs visibly turning on and off, an electric motor may slow down or stop its rotation during the period between mains half-cycles when the thyristor is switched off. Accordingly, the motor may rotate in an unstable, faltering manner, which is obviously undesirable in applications when a steady rotation is required.
Various approaches have been suggested in the past for addressing this problem. For example, U.S. Pat. No. 8,497,637 B2 describes an LED driver circuit, which receives a voltage signal output from a phase-cut triac dimmer. A triac is a particular thyristor-type device allowing bidirectional current flow, which can be triggered by a positive or negative gate current. In U.S. Pat. No. 8,497,637 B2, the output of the triac dimmer is pulse width modulated in the LED driver so that an LED light source is turned on and off at a frequency higher than the threshold of human perception. These LED drivers may be designed to work with standard triac dimmer-switches. However, such an LED driver is not always desirable as additional circuitry is required in a driver circuit at the LED light source. Typically, LED drivers are arranged in close proximity to the LED light source to avoid power losses associated with their high output current. In many cases they are integral within the LED lamp. There is therefore a desire to provide a dimmer-switch, which allows any LED lamp to be fitted rather than LED lamps with special drivers for use with standard dimmer-switches. For multiple lamp loads, the use of such a dimmer-switch incorporating the present invention would replace the need for multiple special drivers.